Beating the coffee ring effect for smooth coatings

Researchers figure out a way to avoid the coffee ring effect, which creates …

Ever wonder why a drop of coffee leaves a ring behind when it dries? Physicists did. In 1997, a group from Chicago came up with a theory of how it works. It turned out to be such a universal theory that it shows up in a number of problems related to deposition of material. Since then physicists have been trying to find a way to get around it and stop making rings. Now a group of physicists in Philadelphia have done it.

So what is the coffee-ring effect? When a drop of coffee dries, its outer edges are pinned, so the radius does not change even as the amount of liquid shrinks. As the volume of the drop decreases from evaporation, the contact angle of the edge of the drop also decreases. This causes a radial capillary flow that carries coffee particles from the center of the drop to the edge, where they are deposited, forming a ring.

What researchers have shown is that the coffee-drop effect can be negated if the particles are not spherical. When ellipsoidal particles are transported to the drop edge, they form loosely packed structures that can resist the capillary flow. When the drop has completely evaporated, these particles are more or less evenly distributed. The more elongated the particles, the more uniform the deposition, providing a way to control the distribution of material.

The coffee-ring effect crops up when dealing with many methods of depositing materials. Having the ability to control the uniformity of deposition will be useful in fields such as coating and printing.

This research project began when the office manager had to re-finish the new mahogany boardroom table three times in six months. Scientists, refusing to believe that it was as simple as "wiping off the bottom of the cup," decided to investigate...

it would be funny if they made a docudrama about the people who discovered this, and sexed it up like the facebook guy movie, or that blackjack scam movie from the college. it could be called the guys who watched coffee droplets dry or something to that effect.

A bunch of coffee beans arranged in a circle could have been just as good for misleading people about the topic of the article. And why stop there? A picture on which a cup of coffee rings the bell? A wedding ring made of coffee? A telephone in the shape of coffee, rings. The picure of xanthine, the rings that form the basis of caffeine. A band of hoodlums using a certain scrpting language.

The ring in the photo is caused by coffee on the bottom of mug. It has absolutely nothing to do with how these droplets form rings. Even calling it "the coffee ring effect" is misleading and confusing, in a lame attempt to be cute, like the science reporting on Good Morning America or Fox and Friends.

Just how many people who post here need to count their fish sticks every day, anyway?

I've never heard that expression before, but man do I like it. I'm going to have to steal that one from you.

Though I have to admit, at first I was thinking that a coaster would be the logical answer as well, but once I got into the article it made perfect sense. I wonder if it was first paid attention to with the stains from coffee and therefore got its name from that...

We once used a photo of Condoleeza Rice, holding her fingers together to indicate something small, as an illustration for a nano tech story. Her fingers were nowhere close to a nanometer apart - more like a centimeter! Horribly misleading....

The only accurate images of a coffee ring of the sort described here that we could find that could be used without threat of a lawsuit were only 175 pixels across, which we cannot use. This is often the case with scientific stories, and so we went with our standard Plan B: find an image that evokes the matter at hand. Conveniently, UPenn had provided a press release with this image, since some of the authors originated there. It's not 100 percent accurate, but it evokes a coffee ring, which is what we were aiming for.

So, i hope that everyone who found the use of an evocative image intolerable will immediately write letters of protest to Nature.

Well, I for one don't read Nature. I do read Ars though...

I'm not saying the photo is horrible, but this photo actually makes it harder to understand the article. Did you consider *gasp* no picture? Or to put it another way, what is the point of having a picture with the story?

The only accurate images of a coffee ring of the sort described here that we could find that could be used without threat of a lawsuit were only 175 pixels across, which we cannot use. This is often the case with scientific stories, and so we went with our standard Plan B: find an image that evokes the matter at hand. Conveniently, UPenn had provided a press release with this image, since some of the authors originated there. It's not 100 percent accurate, but it evokes a coffee ring, which is what we were aiming for.

Psh, what an AT&T response to inquiry! Stop making excuses and pay Aurich so he'll stop striking.

Lighten up, Francises. It's a picture, and it depicts a coffee ring. I'm pretty sure Bill O'Reilly doesn't actually have a creepy anon mask surgically implanted into his face, the National Music Publishers' Association didn't literally raise a white flag, that Leo Apotheker isn't suffering the effects of long-term mercury poisoning, and that there is actually no such thing as a shoe branded "Air Sperm."

The article is perfectly clear as to what it's talking about - obviously, or all you wet blankets wouldn't be in here complaining that the picture doesn't match the actual contents.

But I'll bite. Yes, people still print. I think your definition of "printing" is a bit myopic. While people may be running their home inkjets a bit less these days, printing is a huge part of the manufacturing industry. I would be willing to bet that 90% of all manufactured products include a form of printing somewhere in the process. And that's not even including "coating", which is also mentioned in the article. Look at the highly contested photo with the article? How do you think the U Penn logo got on that cup?

Bringing it back around to the article, I can see this type of data being extremely useful in manufacturing. I work in a screen printing company, and ink laydown is one of the big things our R&D department fights with regularly.

The article is perfectly clear as to what it's talking about - obviously, or all you wet blankets wouldn't be in here complaining that the picture doesn't match the actual contents.

I'm going to disagree, but explain why.

I don't drink coffee. While I've probably seen coffee rings as pictured in the article, I can't say I've ever paid enough attention to them to notice that the ring itself is bordered by darker bands. So, until I saw some of the photos linked in the thread, I was honestly confused as to what the article was about, why coffee rings would be worthy of investigation (beyond "well, the mugs are round, so..."), and how differently-shaped particles would affect their shape significantly.

Dr. Jay wrote:

We once used a photo of Condoleeza Rice, holding her fingers together to indicate something small, as an illustration for a nano tech story. Her fingers were nowhere close to a nanometer apart - more like a centimeter! Horribly misleading....

Yes, and I know what nano-tech is. Even if I hadn't heard the term before, I would have no reason to believe that you were attempting to literally depict nano-tech with a photo of a person (especially one indicating "small" via gesture); the first paragraph of the article would almost certainly have been sufficient to disabuse me of the notion even if I had thought it. Coffee rings are a different beast, though: the photo does, indeed, show a ring of coffee, and the article talks about why coffee forms rings. They appear to go together, and that appearance is sufficient to make the article less clear.